The present invention relates to cleaning of the nozzle surface of an ink writing head in an ink printer, the writing head being of a type provided with nozzles and with variable volume chambers in flow connection with the nozzles for discharging ink in droplet form, and to the use of a particular cleaning medium for that purpose.
Ink writing heads, mosaic writing heads and similar devices for transferring droplets of ink onto a print carrier are disclosed, for example, in German Offenlegungsschrift [Laid-open Application] No. 2,132,082 and counterpart U.S. Pat. No. 3,946,398, German Auslegeschrift [Published Application] No. 2,164,614, U.S. Pat. No. 3,747,120 and German Offenlegungsschrift No. 2,543,451. These writing heads are provided with chambers whose volume can temporarily be changed and which are each in flow connection with a nozzle to which a pulse for discharging a drop of ink is transmitted when the chamber volume is reduced.
According to observations made in the art, the exit area of the nozzle or the nozzle surface, respectively, attracts impurities from the ambient air which remain attached thereto and lead to malfunctions in the dynamic behavior of the ink during the emission of droplets. Ink that has thickened in the nozzle area also leads to changes in the droplet break-off behavior, causing droplets to be deflected and thus transmitted inaccurately to the print carrier. The volume of individual droplets vary and generally becomes less than the volume of a properly transferred droplet. Moreover, such impurities may cause one or a plurality of air bubbles to be captured when the ink meniscus retreats into the nozzle, which can lead to the non-functioning of one or a plurality of nozzles for an extended period of time. The soiling of the nozzle surface may occur when the nozzle is in its operative or its inoperative state.
German Auslegeschrift No. 2,362,576 discloses a device for preventing the discharge of ink from the nozzles of a writing head by means of a cap of a closed, fine-celled and rubber elastic foamed substance which contacts the nozzle surface. By closing the nozzles, it becomes possible to prevent infiltration and trapping of air, discharge of ink from the nozzles, soiling of the nozzles and formation of dried ink in the nozzles during periods when the writing head is not in use. The honeycomb structure of the foamed material is intended to produce a suction effect when the cap is lifted off the nozzle thus carrying along dirt particles that might adhere to the nozzle surface. It has been found, however, that after repeated and extended covering periods, the foamed material loses its elasticity. To produce a seal on the nozzle surface on all sides, the foamed material must be highly deformed by means of a given force when it is applied to the nozzle. Uniform wetting of the nozzle region and of the adjacent nozzle surfaces cannot be attained.
German Auslegeschrift No. 2,317,911 discloses the sucking away of a neutral fluid and once it has been exchanged, also a writing fluid expelled from the nozzles of an ink writing head, by means of a piece of leather. The leather, as long as it is dry, has a contact angle which is greater than 90.degree.. The contact angle can also be reduced by prior moistening and wringing out. A leather sheet can be used in an ink writing mechanism only within limits and only if it is in continuous flow connection with a fluid reservoir.
It has, moreover, been found that, for maintaining the operational efficiency of ink writing heads such as those described, for example in German Auslegeschrift No. 2,164,614, and in U.S. Pat. No. 3,747,120, a distinction must be made between the functions of sealing the nozzle surface from the atmosphere by placing a suitable sealing material against it and cleaning impurities from the nozzle surface by means of a medium suitable for this purpose.
In order to maintain the operational efficiency of an ink writing head over long periods of inactivity, and to prevent the discharge of ink from the nozzles and the entrance of air into the nozzles, drying or thickening of ink in the outer nozzle region, deposits of minute particles from the ambient air, etc. has been found to be necessary to seal the nozzle surface.
On the other hand, as noted above, impurities are deposited on the nozzle surface during the writing process. These impurities must be removed by cleaning the nozzle surface from time to time, preferably while the ink writing head is in a nonprinting area.
German Patent Application No. P 27 05 328.7 discloses a device for covering the nozzle surface of an ink writing head in an ink printer by means of a covering tape of silicone rubber which is driven via guide rollers. When this tape contacts the nozzle surface at the ink writing head, the nozzle surface and the nozzles are closed and sealed under utilization of a viscous, elastic flow. Surface adhesion to the covering tape causes particles to be carried along when the covering tape is removed from the nozzle surface. Likewise, wet particles can be removed from the nozzle region and lifted off the covering tape by means of a scraping device.